U.S. patent application number 13/324413 was filed with the patent office on 2012-06-28 for semiconductor memory device and control method therefor.
Invention is credited to Yasuhiko HONDA.
Application Number | 20120163088 13/324413 |
Document ID | / |
Family ID | 46316606 |
Filed Date | 2012-06-28 |
United States Patent
Application |
20120163088 |
Kind Code |
A1 |
HONDA; Yasuhiko |
June 28, 2012 |
SEMICONDUCTOR MEMORY DEVICE AND CONTROL METHOD THEREFOR
Abstract
According to one embodiment, a semiconductor memory device
includes a memory cell array including memory cells, each of which
is arranged at a position of between a word line and a bit line, a
row decoder, and a bit line control circuit. And when data is to be
read out from the memory cell, a charge control circuit controls
the gate voltages of a first transistor, a second transistor, a
third transistor, and a fourth transistor, respectively, so that
the bit line is charged in accordance with a first characteristic
obtained by increasing a current driving capacity of the first
transistor during a desired period after start of charge of the bit
line, and the bit line is then charged in accordance with a second
characteristic obtained by returning the current driving capacity
of the first transistor to the lower current driving capacity after
elapse of the desired period.
Inventors: |
HONDA; Yasuhiko;
(Hiratsuka-shi, JP) |
Family ID: |
46316606 |
Appl. No.: |
13/324413 |
Filed: |
December 13, 2011 |
Current U.S.
Class: |
365/185.18 |
Current CPC
Class: |
G11C 16/0483 20130101;
G11C 16/26 20130101; G11C 16/24 20130101 |
Class at
Publication: |
365/185.18 |
International
Class: |
G11C 16/26 20060101
G11C016/26 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 22, 2010 |
JP |
2010-286321 |
Claims
1. A semiconductor memory device comprising: a memory cell array
including memory cells, which are arranged in a matrix and
electrically connected to a bit line; and a bit line control
circuit including a first transistor which has a current path with
one end electrically connected to the bit line, and the other end
electrically connected to a first node, a second transistor which
has a current path with one end electrically connected to the first
node, and the other end electrically connected to a power supply
voltage, and fixes a potential of the first node, a third
transistor which has a current path with one end electrically
connected to the first node, and the other end electrically
connected to a second node, and fixes the potential of the first
node, a fourth transistor which has a current path with one end
electrically connected to the second node, and the other end
electrically connected to the power supply voltage, and a charge
control circuit configured to control gate voltages of the first
transistor, the second transistor, the third transistor, and the
fourth transistor, respectively, wherein in a read operation, the
charge control circuit controls the gate voltages of the first
transistor, the second transistor, the third transistor, and the
fourth transistor, respectively, so that the bit line is charged in
accordance with a first current driving capacity of the first
transistor after starting to charge up the bit line, and the bit
line is charged in accordance with a second current driving
capacity of the first transistor, which is lower than the first
current driving capacity after elapse of the desired period.
2. The device of claim 1, wherein the charge control circuit
further controls to set a transition period in which the gate
voltage of the first transistor is dropped to change the first
current driving capacity of the first transistor into the second
current driving capacity of the first transistor, and then the gate
voltage of one of the second transistor, the third transistor, and
the fourth transistor is raised.
3. The device of claim 2, wherein the charge control circuit
further controls to short the gate voltages of the first
transistor, the second transistor, the third transistor, and the
fourth transistor, respectively, during the transition period.
4. The device of claim 1, wherein the charge control circuit
further comprises: a first voltage generation circuit, a second
voltage generation circuit, a third voltage generation circuit, and
a fourth voltage generation circuit which are configured to
generate the gate voltages of the first transistor, the second
transistor, the third transistor, and the fourth transistor,
respectively; and a plurality of first switching elements, each of
which has a current path with one end and the other end
electrically connected in parallel between an output of each of the
first voltage generation circuit, the second voltage generation
circuit, the third voltage generation circuit, and the fourth
voltage generation circuit, and the gate voltage of a corresponding
one of the first transistor, the second transistor, the third
transistor, and the fourth transistor.
5. The device of claim 4, wherein the charge control circuit
further comprises: a second switching element having a current path
with one end and the other end electrically connected between an
input of the gate voltage of the first transistor and an input of
the gate voltage of the second transistor; a third switching
element having a current path with one end and the other end
electrically connected between the input of the gate voltage of the
second transistor and an input of the gate voltage of the third
transistor; and a fourth switching element having a current path
with one end and the other end electrically connected between the
input of the gate voltage of the third transistor and an input of
the gate voltage of the fourth transistor.
6. The device of claim 5, wherein the charge control circuit
controls to turn off the first switching elements, and to turn on
the second switching element, the third switching element, and the
fourth switching element, during the transition period.
7. The device of claim 4, wherein the charge control circuit
controls to turn on all of the first switching elements during the
transition period.
8. A method for controlling a read operation of a semiconductor
memory device comprising a memory cell array including memory
cells, which are arranged in a matrix and electrically connected to
a bit line, and a bit line control circuit including a first
transistor which has a current path with one end electrically
connected to the bit line, and the other end electrically connected
to a first node, a second transistor which has a current path with
one end electrically connected to the first node, and the other end
electrically connected to a power supply voltage, and fixes a
potential of the first node, a third transistor which has a current
path with one end electrically connected to the first node, and the
other end electrically connected to a second node, and fixes the
potential of the first node, a fourth transistor which has a
current path with one end electrically connected to the second
node, and the other end electrically connected to the power supply
voltage, and a charge control circuit configured to control gate
voltages of the first transistor, the second transistor, the third
transistor, and the fourth transistor, respectively, the method
comprising: charging the bit line in accordance with a first
current driving capacity of the first transistor after starting to
charge the bit line; and charging the bit line in accordance with a
second current driving capacity of the first transistor, which is
lower than the first current driving capacity after elapse of the
desired period.
9. The method of claim 8, further comprising: setting a transition
period in which the gate voltage of the first transistor is dropped
to change the first current driving capacity of the first
transistor into the second current driving capacity of the first
transistor, and then the gate voltage of one of the second
transistor, the third transistor, and the fourth transistor is
raised.
10. The method of claim 9, further comprising: shorting the gate
voltages of the first transistor, the second transistor, the third
transistor, and the fourth transistor, respectively, during the
transition period.
11. The method of claim 8, wherein the charge control circuit
further comprises: a first voltage generation circuit, a second
voltage generation circuit, a third voltage generation circuit, and
a fourth voltage generation circuit which are configured to
generate the gate voltages of the first transistor, the second
transistor, the third transistor, and the fourth transistor,
respectively; and a plurality of first switching elements, each of
which has a current path with one end electrically and the other
end electrically connected in parallel between an output of each of
the first voltage generation circuit, the second voltage generation
circuit, the third voltage generation circuit, and the fourth
voltage generation circuit, and the gate voltage of a corresponding
one of the first transistor, the second transistor, the third
transistor, and the fourth transistor.
12. The method of claim 11, wherein the charge control circuit
further comprises: a second switching element having a current path
with one end electrically and the other end electrically connected
between an input of the gate voltage of the first transistor and an
input of the gate voltage of the second transistor; a third
switching element having a current path with one end electrically
and the other end electrically connected between the input of the
gate voltage of the second transistor and an input of the gate
voltage of the third transistor; and a fourth switching element
having a current path with one end electrically and the other end
electrically connected between the input of the gate voltage of the
third transistor and an input of the gate voltage of the fourth
transistor.
13. The method of claim 12, wherein the charge control circuit
controls to turn off the first switching elements, and to turn on
the second switching element, the third switching element, and the
fourth switching element, during the transition period.
14. The method of claim 11, wherein the charge control circuit
controls to turn on the first switching elements during the
transition period.
15. The device of claim 1, wherein the first to third transistors
include charging capacities lower than a charging capacity of the
fourth transistor.
16. The method of claim 8, wherein the first to third transistors
include charging capacities lower than the charging capacity of the
fourth transistor.
17. The device of claim 1, wherein the first to third transistors
include gate lengths greater than a gate length of the fourth
transistor.
18. The method of claim 8, wherein the first to third transistors
include gate lengths greater than a gate length of the fourth
transistor.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2010-286321,
filed Dec. 22, 2010, the entire contents of which are incorporated
herein by reference.
FIELD
[0002] Embodiments described herein relate generally to, for
example, a semiconductor memory device and a control method
therefor.
BACKGROUND
[0003] Conventionally, the read characteristic of a NAND flash
memory which exemplifies a semiconductor memory device is mainly
determined by charge of bit lines. In such a NAND flash memory,
with advances in micropatterning of the manufacturing process, the
decrease in memory cell current and the mutual interference between
adjacent bit lines become conspicuous, so the read characteristic
may degrade.
[0004] Also, a read operation is performed in verify at the time of
a write or erase operation in or from the NAND flash memory as
well, so the write or erase characteristic may similarly
degrade.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a block diagram illustrating an example of the
entire arrangement of a semiconductor memory device according to
the first embodiment;
[0006] FIG. 2 is an equivalent circuit diagram illustrating an
example of a memory cell array and its peripheral circuit shown in
FIG. 1;
[0007] FIG. 3 is an equivalent circuit diagram illustrating an
arrangement shown in FIG. 2;
[0008] FIG. 4 is a graph illustrating the relationship between time
and the bit line potential in data read;
[0009] FIG. 5 is a graph illustrating the relationship between time
and the sense amplifier current in data read;
[0010] FIG. 6 is a graph showing the load characteristic according
to a Comparative Example;
[0011] FIG. 7 is a graph showing the load characteristic according
to the first embodiment;
[0012] FIG. 8 is an equivalent circuit diagram illustrating an
example of the arrangement of a bit line control circuit according
to the first embodiment;
[0013] FIG. 9 is an equivalent circuit diagram illustrating an
example of the arrangement of a charge control circuit according to
the first embodiment;
[0014] FIG. 10 is a timing chart showing a sense operation in data
read according to the first embodiment;
[0015] FIG. 11 is a circuit diagram showing the voltage
relationship of the charge control circuit in a charge operation
according to the first embodiment;
[0016] FIG. 12 is a timing chart showing a sense operation in data
read according to the second embodiment;
[0017] FIG. 13 is a circuit diagram showing the voltage
relationship of a charge control circuit in a charge operation
according to the second embodiment;
[0018] FIG. 14 is a graph for explaining the load characteristic
according to the third embodiment;
[0019] FIG. 15 is an equivalent circuit diagram illustrating an
example of the arrangement of a bit line control circuit according
to the third embodiment;
[0020] FIG. 16 is an equivalent circuit diagram illustrating an
example of the arrangement of a charge control circuit according to
the third embodiment;
[0021] FIG. 17 is a timing chart showing a sense operation in data
read according to the third embodiment;
[0022] FIG. 18 is a circuit diagram showing the voltage
relationship of the charge control circuit in a charge operation
according to the third embodiment;
[0023] FIG. 19 is a graph showing the load characteristic according
to the third embodiment; and
[0024] FIG. 20 is a circuit diagram showing the voltage
relationship of a charge control circuit in the EQL period
according to the first modification.
DETAILED DESCRIPTION
[0025] In general, according to one embodiment, a semiconductor
memory device includes a memory cell array including memory cells,
each of which is arranged at a position corresponding to the
intersection point between a word line and a bit line, a row
decoder which applies a read voltage to the word line, and a bit
line control circuit including a first transistor which has a
current path with its one end connected to the bit line, and its
other end connected to a first node, a second transistor which has
a current path with its one end connected to the first node, and
its other end connected to a power supply voltage, and fixes the
potential of the first node, a third transistor which has a current
path with its one end connected to the first node, and its other
end connected to a second node, and fixes the potential of the
first node, a fourth transistor which has a current path with its
one end connected to the second node, and its other end connected
to the power supply voltage, and a charge control circuit which
controls the gate voltages of the first transistor, the second
transistor, the third transistor, and the fourth transistor,
respectively. When data is to be read out from the memory cell, the
charge control circuit controls the gate voltages of the first
transistor, the second transistor, the third transistor, and the
fourth transistor, respectively, so that the bit line is charged in
accordance with a first characteristic obtained by increasing the
current driving capacity of the first transistor during a desired
period after the start of charge of the bit line, and the bit line
is then charged in accordance with a second characteristic obtained
by returning the current driving capacity of the first transistor
to the lower current driving capacity after the elapse of the
desired period.
[0026] Embodiments will be described below with reference to the
accompanying drawings. A semiconductor memory device will be
described in detail hereinafter by taking a NAND flash memory as an
example. Note that in the following description, the same reference
numerals denote the same parts throughout the drawings.
First Embodiment
[0027] A semiconductor memory device according to the first
embodiment will be described first with reference to FIGS. 1, 2, 3,
4, 5, 6, 7, 8, 9, 10 and 11.
1. Arrangement Example
1-1. Example of Entire Arrangement
[0028] An example of the entire arrangement of a NAND flash memory
100 which exemplifies a semiconductor memory device according to
the first embodiment will be described first with reference to FIG.
1.
[0029] The NAND flash memory 100 includes a memory cell array 1,
bit line control circuit 2, column decoder 3, data input/output
buffer 4, data input/output terminal 5, row decoder 6, control
circuit 7, control signal input terminal 8, source line control
circuit 9, and well control circuit 10, as shown in FIG. 1.
[0030] The memory cell array 1 includes a plurality of bit lines, a
plurality of word lines, a source line SRC, and a plurality of
memory cells, as will be described later. The plurality of memory
cells are arranged at positions corresponding to the intersection
points between the bit lines and the word lines. The memory cell
array 1 is formed from a plurality of blocks BLK. In the blocks
BLK, electrically rewritable memory cells are arranged in a matrix.
Each memory cell has, for example, a stacked structure including a
control gate electrode and a charge storage layer (for example, a
floating gate electrode), and stores binary or multi-level data in
accordance with a change in threshold of a transistor, which is
determined by the amount of charge injected into the charge storage
layer. Note that each memory cell may have a MONOS
(Metal-Oxide-Nitride-Oxide-Silicon) structure which traps electrons
in a nitride film.
[0031] The memory cell array 1 is electrically connected to the bit
line control circuit 2 used to control the voltages of bit lines
BL, and the row decoder 6 used to control the voltages of word
lines WL.
[0032] The bit line control circuit 2 includes data storage
circuits and a charge control circuit (neither is shown). The data
storage circuits serves both as sense amplifiers which sense and
amplify the voltages of the bit lines in the memory cell array 1,
and data latch circuits which latch data. Also, the charge control
circuit controls a clamp NMOS transistor (to be described later).
Note that the bit line control circuit 2 will be described in
detail later.
[0033] The bit line control circuit 2 reads out data from each
memory cell via a corresponding bit line, detects the state of the
memory cell via the bit line, or writes data in the memory cell by
applying a write voltage to the memory cell via the bit line. Also,
the bit line control circuit 2 is electrically connected to the
column decoder 3 and data input/output buffer 4. A data storage
circuit in the bit line control circuit 2 is selected by the column
decoder 3. Data read out from each memory cell to the data storage
circuit is output to the outside from the data input/output
terminal 5 via the data input/output buffer 4.
[0034] Write data which is externally input to the data
input/output terminal 5 is stored in the data storage circuit,
which is selected by the column decoder 3, via the data
input/output buffer 4.
[0035] The data input/output terminal 5 receives not only write
data but also addresses and various commands such as write, read,
erase, and status read commands.
[0036] The row decoder 6 is electrically connected to the memory
cell array 1. The row decoder 6 applies voltages necessary for a
read operation, a write operation, or an erase operation to the
word lines and select gate lines in the memory cell array 1. Also,
the row decoder 6 includes a driver control circuit connected to a
plurality of control lines (to be described later) so as to
selectively apply voltages to the word lines and select gate lines.
In, for example, a data erase operation, one of the plurality of
blocks BLK is selected by the row decoder 6, and the remaining
blocks are kept unselected.
[0037] The source line control circuit 9 is electrically connected
to the memory cell array 1. The source line control circuit 9
controls the voltage of the source line.
[0038] The well control circuit 10 is electrically connected to the
memory cell array 1. The well control circuit 10 controls the
voltage of a semiconductor substrate (well) on which memory cells
are to be formed. In this embodiment, the memory cell array 1
adopts a double well structure, which has an N well (Well(N))
formed on a semiconductor substrate, and the plurality of blocks
BLK arranged in a P well (Well(P)) formed on the N well.
[0039] The control circuit 7 controls the memory cell array 1, bit
line control circuit 2, column decoder 3, data input/output buffer
4, row decoder 6, source line control circuit 9, and well control
circuit 10. The control circuit 7 includes a boosting circuit (not
shown) which boosts the power supply voltage. The control circuit 7
causes the boosting circuit to boost the power supply voltage as
needed, and supplies the boosted voltage to the bit line control
circuit 2, column decoder 3, data input/output buffer 4, row
decoder 6, source line control circuit 9, and well control circuit
10. The control circuit 7 performs control operations in accordance
with control signals (for example, a command latch enable signal
CLE, an address latch enable signal ALE, and a ready/busy signal
RY/BY) which are externally input via the control signal input
terminal 8, and commands input via the data input/output buffer 4
from the data input/output terminal 5. That is, in programming,
verifying, reading, or erasing data, the control circuit 7
generates desired voltages and supplies them to the units of the
memory cell array 1, in accordance with control signals and
commands, thereby controlling the operation of the overall NAND
flash memory 100.
1-2. Memory Cell Array and its Peripheral Circuit
[0040] An example of the arrangement of the memory cell array and
its peripheral circuit according to the first embodiment will be
described next with reference to FIG. 2.
[0041] FIG. 2 illustrates an example of the detailed arrangement of
the memory cell array 1, bit line control circuit 2, and row
decoder 6.
[0042] The memory cell array 1 includes blocks BLK0, BLK1, . . . ,
BLKn formed by juxtaposing NAND strings each including a plurality
of memory cells MC having current paths connected in series with
each other. Each NAND string is formed from n (for example, 64)
memory cells MC having current paths connected in series with each
other, and has its one end connected to a drain-side select MOS
transistor SGD, and its other end connected to a source-side select
MOS transistor SGS. The source-side select MOS transistor SGS is
connected to the source line SRC.
[0043] The control gate electrodes of the memory cells MC arranged
on each row are connected to word lines WL0 to WLn, respectively.
Note that for the sake of simplicity, FIG. 2 illustrates only the
word lines WL0, WL1, WLn-1, WLn, and WLn-1, and do not show word
lines between them. A plurality of word lines will simply be
referred to as the word lines WL if they require no particular
distinction. The gate of the drain-side select MOS transistor SGD
is connected to a drain-side select gate line VSGD. The gate of the
source-side select MOS transistor SGS is connected to a source-side
select gate line VSGS.
[0044] The row decoder 6 includes a driver circuit 6-1, a plurality
of transfer NMOS transistors TSGS, TSGD, and TWL0 to TWLn
corresponding to the respective blocks BLK, and a plurality of
control lines G_VSGS, G_VSGD, G_WL0, G_WL1, . . . , G_WLn-1, and
G_WLn. The drains of the transfer NMOS transistors TSGS and TSGD
are connected to the source-side select gate line VSGS and
drain-side select gate line VSGD, respectively. The drains of the
transfer NMOS transistors TWL0 to TWLn are connected to the word
lines WL0 to WLn, respectively, which are connected to the control
gates of corresponding sets of memory cells MC.
[0045] The sources of the transfer NMOS transistors TSGS, TSGD, and
TWL0 to TWLn are connected to the control lines G_VSGS, G_VSGD,
G_WL0, G_WL1, . . . , G_WLn-1, and G_WLn, respectively, which are
connected to the driver circuit 6-1. A block selection signal
corresponding to an external address is input to the gates of the
transfer NMOS transistors TSGS, TSGD, and TWL0 to TWLn. The driver
circuit 6-1 controls the gate voltages and source voltages of the
transfer NMOS transistors TSGS, TSGD, and TWL0 to TWLn in
accordance with the output from the control circuit 7. That is, the
row decoder 6 causes the driver circuit 6-1 to control the gate
voltages and source voltages of the transfer NMOS transistors to
select an arbitrary block BLK in the memory cell array 1, thereby
executing a write or read operation in the selected block BLK.
[0046] Bit lines BL0, BL1, and BL2 are arranged perpendicularly to
the word lines WL0 to WLn and source line SRC. Although FIG. 2
illustrates only the three bit lines BL0, BL1, and BL2 as a
detailed example, the number of bits is not limited to this, as a
matter of course. An arbitrary total number of bit lines, for
example, 2,048 bit lines can be juxtaposed in one block BLK.
[0047] The bit line control circuit 2 includes a plurality of sense
amplifiers SA0, SA1, and SA2, clamp transistors NMOS0 to NMOS2, and
a charge control circuit 2-1.
[0048] The sense amplifiers SA0, SA1, and SA2 are connected to the
bit lines BL0, BL1, and BL2 via the clamp transistors NMOS0 to
NMOS2, respectively. Sense amplifiers SA sense or control the
potentials of the bit lines BL. The charge control circuit 2-1
controls the gate voltages of the clamp transistors NMOS0 to NMOS2,
and the sense amplifiers SA0, SA1, and SA2 to control, for example,
charge of the bit lines BL0, BL1, and BL2. The arrangement and
operation of the charge control circuit 2-1 will be described in
detail later.
1-3. Phenomenon in which Read Characteristic Degrades Due to Mutual
Interference with Adjacent Bit Line
[0049] A phenomenon in which the read characteristic degrades due
to mutual interference with an adjacent bit line will be described
next with reference to FIGS. 3, 4, 5, 6 and 7.
[0050] FIG. 3 shows the three adjacent bit lines BL0, BL1, and BL2,
the clamp transistors NMOS0 to NMOS2 used to clamp the potentials
of these bit lines, respectively, and the sense amplifiers SA0,
SA1, and SA2 used to sense the potentials of these bit lines,
respectively. The gates of the clamp transistors NMOS0 to NMOS2 are
controlled by a gate signal Clamp_D. Reference symbols ISA0 to ISA2
denote the currents flowing from the sense amplifiers SA0 to SA2,
respectively; and Icell0 to Icell2, the currents flowing through
memory cells MC0 to MC2, respectively.
[0051] The adjacent bit lines BL0 and BL1 and the adjacent bit
lines BL1 and BL2 have adjacent parasitic capacitances between
them. Note that bit lines adjacent to the adjacent bit lines BL
have parasitic capacitances between them as well.
[0052] A data read operation by the semiconductor memory device
having the above-mentioned arrangement when the memory cell MC1
connected to the bit line BL1 is turned on, and the memory cells
MC0 and MC2 connected to the bit lines BL0 and BL2, respectively,
are turned off will be described.
[0053] FIGS. 4 and 5 show a change in bit line potential and a
change in current flowing from each sense amplifier, respectively,
in the data read operation in that case. FIG. 4 illustrates the
relationship between time and the bit line potential in the data
read operation. FIG. 5 illustrates the relationship between time
and the sense amplifier current in the data read operation.
[0054] As is obvious from FIG. 5, the current "ISA1" becomes
smaller (less) than the current "Icell1". On the other hand, as is
obvious from FIG. 5 as well, the current "ISA0,2" is always larger
(more) than the current "Icell0,2". The operating point of charge
of the bit lines BL0 and BL2 connected to the memory cells MC0 and
MC2, respectively, in an OFF state is highest, so their charge time
is long. Also, the potential of the bit line BL1 connected to the
memory cell MC1 in an ON state rises due to the parasitic
capacitance between the adjacent bit lines BL0 and BL2, so the
apparent memory cell current ISA1 when viewed from the sense
amplifier SA1 decreases.
[0055] In this manner, the inventor observed the following
phenomenon. That is, with miniaturization of memory cells, the
memory cell current in an ON state decreases, and the increase in
parasitic capacitance between adjacent bit lines becomes
conspicuous. As a result, it takes a long time to stabilize the
currents of both a memory cell in an ON state and a memory cell in
an OFF state, when viewed from the sense amplifiers.
[0056] In this case, the fact that the current (for example, the
current ISA1) of a specific memory cell decreases reveals that its
corresponding bit line requires charging up to a sufficiently high
level. Also, the fact that the coupling between adjacent bit lines
increases reveals that an ON cell (selected cell MC1) with a low
operating point and an OFF cell (unselected cell MC0 or MC2) with a
high operating point interfere with each other so as to delay
arrival at the operating points. This phenomenon becomes
conspicuous when a comparison is made with an extreme case in which
adjacent bit lines have no coupling.
[0057] Further, with miniaturization of memory cells, the amount of
interference increases due to the above-mentioned coupling.
Load Characteristic
[0058] A mechanism of a change in bit line potential according a
Comparative Example will be described next with reference to a load
characteristic graph shown in FIG. 6. As shown in FIG. 6, the
potential of the bit line BL changes in accordance with the
characteristic line (N0) of the NMOS transistor NMOS0 which clamps
the potential of the bit line BL, and reaches an operating point
that is the intersection point with a memory cell current Icell.
Upon reaching the operating point, the currents of the memory cells
in an OFF state and an ON state stabilize, and therefore reach an
equilibrium, as shown in FIG. 6. In other words, the load
characteristic (N0) of this NMOS transistor becomes a factor which
determines the bit line charging capacity.
[0059] A mechanism of a change in bit line potential according to
the first embodiment will be described next with reference to a
load characteristic graph shown in FIG. 7. As shown in FIG. 7, to
improve the bit line charging capacity, in this embodiment, first,
during a desired period (to be also referred to as the initial
charge period hereinafter) after the start of charge of the bit
line BL (this time is indicated by t0 in FIG. 7), the bit line BL
is rapidly charged in accordance with the load characteristic (N1:
first characteristic) of the clamp NMOS transistor NMOS0, which
expresses a charging capacity (current driving capacity) higher
than that expressed by the load characteristic (N0).
[0060] On the other hand, after the elapse of the desired period
(this time will also be referred to as the end of charge and is
indicated by t1 in FIG. 7), the charging capacity (current driving
capacity) of the clamp transistor NMOS0 is returned to the lower
charging capacity to charge the bit line BL in accordance with, for
example, the same load characteristic (N0: second characteristic)
of the NMOS transistor as in the Comparative Example. As a result,
control is done so that the operating point is determined by the
load characteristic (N0: second characteristic) of the NMOS
transistor.
[0061] This makes it possible to provide a semiconductor memory
device and a control method therefor which can charge each bit line
at high speed by improving the charging capacity of the bit line
while maintaining a given operating point of the bit line. Details
will be described later.
1-4. Bit Line Control Circuit
[0062] An example of the arrangement of a bit line control circuit
according to the first embodiment will be described next with
reference to FIG. 8. FIG. 8 illustrates details of the internal
arrangement of the sense amplifier SA0. Note that for the sake of
descriptive simplicity, the sense amplifiers SA1 and SA2 have the
same internal arrangement as the sense amplifier SA0.
[0063] The bit line control circuit 2 includes the clamp
transistors NMOS0, NMOS1, and NMOS2 which fix the potentials of the
bit lines BL, and the sense amplifiers SA0, SA1, and SA2.
[0064] The charge control circuit 2-1 controls gate signals
Clamp_D, Clamp_E, Clamp_F, and SW under the control of the control
circuit 7. Thus, the bit line BL0 is charged. The charge control
circuit 2-1 detects and checks the memory cell current Icell
flowing through the memory cell MC. Therefore, the gate signals
Clamp_D, Clamp_E, and Clamp_F are input to the gates of clamp
transistors NMOS0, NMOS0a, and NMOS0b, respectively. Also, the gate
signal SW is input to the gate of a switch transistor NMOS0c.
[0065] The clamp transistor NMOS0 (first transistor) has a function
of clamping the potential of the bit line BL under the control of
the charge control circuit 2-1. The clamp transistor NMOS0 has a
current path with its one end connected to the bit line BL0, and
its other end connected to a node node_sa in the sense amplifier
SA0, and has its gate supplied with the gate signal Clamp_D.
[0066] The sense amplifier SA0 includes the clamp transistors
NMOS0a and NMOS0b, the switch transistor NMOS0c, and an inverter
inv0.
[0067] The clamp transistor NMOS0a (second transistor) has a
function of fixing the potential of the node node_sa under the
control of the charge control circuit 2-1. The clamp transistor
NMOS0a has a current path with its one end connected to the node
node_sa, and its other end connected to a power supply voltage Vdd,
and has its gate supplied with the gate signal Clamp_E.
[0068] The clamp transistor NMOS0b (third transistor) has a
function of fixing the potential of the node node_sa under the
control of the charge control circuit 2-1. The clamp transistor
NMOS0b has a current path with its one end connected to the node
node_sa, and its other end connected to a node node_sen, and has
its gate supplied with the gate signal Clamp_F.
[0069] The switch transistor NMOS0c (fourth transistor) has a
function of charging and discharging a capacitor CAP0 under the
control of the charge control circuit 2-1. The switch transistor
NMOS0c has a current path with its one end connected to the node
node_sen, and its other end connected to the power supply voltage
Vdd, and has its gate supplied with the gate signal SW.
[0070] The inverter inv0 has its input connected to the node
node_sen, and its output connected to an output terminal OUT0 of
the bit line control circuit 2.
[0071] One end of the electrode of the capacitor CAP0 is connected
to the node node_sen. The capacitor CAP0 is charged to have a power
supply voltage vdd when the switch transistor NMOS0c is ON, and it
is charged to have the cell voltage of the memory cell MC0 when the
switch transistor NMOS0c is OFF.
[0072] Note that the clamp transistors NMOS0, NMOS0a, and NMOS0b
use transistors having gate lengths larger than that of the switch
transistor NMOS0c to suppress a variation in threshold. Also, the
clamp transistors NMOS0, NMOS0a, and NMOS0b have the same size.
This means that the clamp transistors NMOS0, NMOS0a, and NMOS0b
have charging capacities Dr0, Dra, and Drb lower than the charging
capacity Drc of the switch transistor NMOS0c (charging capacities:
Dr0, Dra, Drb<Drc).
[0073] On the other hand, the switch transistor NMOS0c has a switch
function. The switch transistor NMOS0c has a gate length smaller
than those of the clamp transistors NMOS0, NMOS0a, and NMOS0b. The
switch transistor NMOS0c has a charging capacity Drc higher than
the charging capacities Dr0, Dra, and Drb of the clamp transistors
NMOS0, NMOS0a, and NMOS0b, respectively.
1-5. Charge Control Circuit
[0074] An example of the arrangement of the charge control circuit
2-1 according to the first embodiment will be described next with
reference to FIG. 9. The charge control circuit 2-1 according to
this embodiment includes first to fourth voltage generation
circuits 21 to 24, and switching elements S21-1 to S24-2, as shown
in FIG. 9.
[0075] The first to fourth voltage generation circuits 21 to 24
generate predetermined voltages (for example, an ON voltage Vthn
for the clamp transistor NMOS0).
[0076] The switching elements S21-1 to S24-2 have their gates
supplied with control signals SW1-1 to SW4-2, respectively, from
the control circuit 7 to control conduction/nonconduction of their
current paths.
[0077] The current path of the switching element S21-1 has its one
end connected to the output of the first voltage generation circuit
21, and its other end serving as a terminal which outputs the gate
signal Clamp_D of the clamp transistor NMOS0. The current path of
the switching element S21-2 has its one end connected to the output
of the second voltage generation circuit 22, and its other end
serving as a terminal which outputs the gate signal Clamp_D of the
clamp transistor NMOS0.
[0078] The current path of the switching element S22-1 has its one
end connected to the output of the first voltage generation circuit
21, and its other end serving as a terminal which outputs the gate
signal Clamp_E of the clamp transistor NMOS0a. The current path of
the switching element S22-2 has its one end connected to the output
of the second voltage generation circuit 22, and its other end
serving as a terminal which outputs the gate signal Clamp_E of the
clamp transistor NMOS0a.
[0079] The current path of the switching element S23-1 has its one
end connected to the output of the first voltage generation circuit
21, and its other end serving as a terminal which outputs the gate
signal Clamp_F of the clamp transistor NMOS0b. The current path of
the switching element S23-2 has its one end connected to the output
of the second voltage generation circuit 22, and its other end
serving as a terminal which outputs the gate signal Clamp_F of the
clamp transistor NMOS0b.
[0080] The current path of the switching element S24-1 has its one
end connected to the output of the first voltage generation circuit
21, and its other end serving as a terminal which outputs the gate
signal SW of the switch transistor NMOS0c. The current path of the
switching element S24-2 has its one end connected to the output of
the fourth voltage generation circuit 24, and its other end serving
as a terminal which outputs the gate signal SW of the switch
transistor NMOS0c.
2. Data Read Operation
[0081] A data read operation (of a sense scheme according to this
embodiment) by the sense amplifier SA will be described next with
reference to FIGS. 10 and 11. This description will be given with
reference to a timing chart shown in FIG. 10.
[0082] As shown in FIG. 10, at time t0 at which charge of the bit
line BL starts, the charge control circuit 2-1 raises the potential
of the gate signal Clamp_D of the clamp transistor NMOS0 to
(vsax+vthn) or (vsac+vthn), raises the potential of the gate signal
Clamp_E of the clamp transistor NMOS0a to (vblc+vthn), raises the
potential of the gate signal Clamp_F of the clamp transistor NMOS0b
to (vsax+vthn), and raises the potential of the gate signal SW of
the switch transistor NMOS0c to (vblc+vthn).
[0083] The switching element S22-1 at this time is as shown in FIG.
11. "H" level is input to the control signals SW1-1, SW2-2, SW3-1,
and SW4-2 from the control circuit 7, and "L" level is input to the
control signals SW1-2, SW2-1, SW3-2, and SW4-1 from the control
circuit 7. As a result, the switching elements S21-1, S22-2, S23-1,
and S24-1 are turned off, and the switching elements S21-2, S22-1,
S23-2, and S24-1 are turned on. Therefore, the gate signals
Clamp_D, Clamp_E, Clamp_F, and SW can have a voltage relationship
as mentioned above.
[0084] Note that as shown in FIG. 7, while the potential of the bit
line BL depends on the driving capacity of the NMOS transistor
NMOS0 which clamps the bit line potential, that is, during the
initial charge period of the bit line BL from time t0 in this
embodiment, the charging capacity of the clamp transistor NMOS0 is
driven to a maximum degree (gate signal Clamp_D: vsax+vthn,
vsac+vthn) to rapidly charge the bit line BL in accordance with the
load characteristic (N1) of the transistor NMOS0. This makes it
possible to enhance the charging capacity so as to rapidly charge
the bit line BL.
[0085] After the elapse of a desired period (t1), the charge
control circuit 2-1 drops the potential of the gate signal Clamp_D
of the clamp transistor NMOS0 to (vblc+vthn), raises the potential
of the gate signal Clamp_E of the clamp transistor NMOS0a to
(vsac+vthn), maintains the potential of the gate signal Clamp_F of
the clamp transistor NMOS0b, and raises the potential of the gate
signal SW of the switch transistor NMOS0c to (vdd+vthn).
[0086] In this manner, at the end of charge at time t1, the bit
line BL is charged by setting the charging capacity of the clamp
transistor NMOS0 so as to obtain the load characteristic (N0) of
the NMOS transistor. As a result, control is done so that the
operating point is determined by the load characteristic (N0) of
the NMOS transistor. This makes it possible to provide a
semiconductor memory device and a control method therefor which can
charge each bit line at high speed by improving the charging
capacity of the bit line while maintaining a given operating point
of the bit line.
[0087] For example, as is obvious from a solid line in FIG. 10, the
potential of the bit line BL0 is charged more rapidly in this
embodiment than when control according to this embodiment is not
performed (broken line: Comparative Example). On the other hand, at
the end of charge at time t1, the operating point of the bit line
BL0 is maintained at vblc, as in the Comparative Example. The
Comparative Example shown in FIG. 10 means herein a case in which,
for example, no charge control circuit 2-1 according to this
embodiment is provided, so no control operation according to this
embodiment is performed.
[0088] As shown in FIG. 10, at time t2, when the memory cell
current Icell.degree. supplied by the memory cell MC0 is to be
sensed after the bit line BL0 is charged, the charge control
circuit 2-1 raises the gate signal SW to turn off the switch
transistor NMOS0c.
[0089] At time t3 a predetermined period (sense period T0) after
time t2, the charge control circuit 2-1 drops the gate signal
Clamp_F to turn off the clamp transistor NMOS0b. As a result, the
potential of the node node_sen is determined, and a sense operation
is then performed.
3. Effect
[0090] As described above, according to the semiconductor memory
device and the control operation therefor according to the first
embodiment, at least effect (1) can be obtained:
[0091] (1) It is possible to provide a semiconductor memory device
and a control method therefor which can charge each bit line at
high speed by improving the charging capacity of the bit line while
maintaining a given operating point of the bit line.
[0092] As described above, the charge control circuit 2-1 provided
in the semiconductor memory device according to this embodiment
controls the gate voltage of the first transistor so that the bit
line BL is charged in accordance with the first characteristic (N1)
obtained by increasing the charging capacity (current driving
capacity) of the first transistor NMOS0 at the start of a bit line
charge operation (for example, time t0) in a data read operation
from each memory cell, and the bit line BL is then charged in
accordance with the second characteristic (N0) obtained by
returning the charging capacity (current driving capacity) of the
first transistor NMOS0 to the lower charging capacity at the end of
the charge operation (for example, time t1).
[0093] For example, as shown in the load characteristic graph of
FIG. 7, first, at the start of a charge operation (time t0), the
bit line BL is rapidly charged in accordance with the NMOS
characteristic (N1: first characteristic) obtained by temporarily
increasing the charging capacity (current driving capacity) of the
clamp transistor NMOS0. On the other hand, at the end of the charge
operation (time t1), the bit line BL is charged in accordance with
the same NMOS characteristic (N0: second characteristic) as in, for
example, the Comparative Example by returning the charging capacity
(current driving capacity) of the clamp transistor NMOS0 to the
lower charging capacity, thereby controlling so that the final
operating point is determined by the NMOS characteristic (N0:
second characteristic).
Second Embodiment
Example in which Excessive Charge of Bit Line is Suppressed
[0094] A semiconductor memory device and a control operation
therefor according to the second embodiment will be described next
with reference to FIGS. 12, 13, 14 and 15. This embodiment relates
to an example in which excessive charge of a bit line BL is
suppressed in returning the NMOS characteristic of a first
transistor NMOS0 for clamping to a second characteristic (N1). A
detailed description of the same parts as in the first embodiment
will not be given hereinafter.
<NMOS Load Characteristic>
[0095] The NMOS load characteristic obtained upon making a return
to the second characteristic (N1) will be described first.
[0096] As shown in FIG. 7, in the first embodiment, in the
beginning of charge (time t0), the potential of the bit line BL is
rapidly charged in accordance with the first characteristic (N0) by
temporarily improving the NMOS characteristic of the first
transistor NMOS0 for clamping. On the other hand, at the subsequent
end of charge (time t1), the NMOS characteristic of the first
transistor NMOS0 for clamping is returned to the second
characteristic (N1) to obtain the lower charging capacity. This
makes it possible to rapidly charge the bit line BL so as to
improve the operating characteristic and thereby speed up the
operation, and to guarantee the final operating point of the bit
line BL.
[0097] However, the first embodiment does not take into
consideration the details of the operation for returning the NMOS
characteristic from the first characteristic (N0) to the second
characteristic (N1) at the end of charge (time t1).
[0098] For example, if the potential of the gate signal Clamp_E or
SW becomes higher than the potential vblc+vthn before the potential
of the gate signal Clamp_D changes to the potential vblc+vthn upon
operating the gate signals Clamp_D, Clamp_E, and SW at once, the
bit line BL is charged excessively. This makes it necessary to
discharge the bit line BL by the amount of excessive charge, so it
may take an additional time to stabilize the current when viewed
from the sense amplifier SA.
[0099] Hence, in the second embodiment, gate signals Clamp_D,
Clamp_E, and SW are operated in the following way:
[0100] (i) First, the potential of the gate signal Clamp_D of the
first transistor NMOS0 for clamping is changed to a potential
higher than a potential vblc+vthn to charge a bit line BL0, and the
load characteristic of the NMOS transistor is returned to the
second characteristic (N0), thereby ensuring a given operating
point of the bit line BL.
[0101] (ii) Subsequently, the gate potentials of remaining second
to fourth transistors NMOS0a, NMOS0b, and NMOS0c are changed to
higher gate potentials. Note that a case in which a gate signal
Clamp_F of the second transistor remains the same will be taken as
an example in this embodiment. The gate potential of at least one
of the second to fourth transistors NMOS0a, NMOS0b, and NMOS0c need
only be lower than that of the first transistor NMOS0 before the
gate potentials of the second to fourth transistors NMOS0a, NMOS0b,
and NMOS0c change to gate potentials higher than that of the first
transistor NMOS0.
[0102] In other words, a transition period T1 is set between the
operations defined in (i) and (ii). With this control, the
potentials of the second to fourth transistors other than the first
transistor NMOS0 can be prevented from becoming higher than the
potential vblc+vthn of the gate signal Clamp_D of the first
transistor NMOS0 before the potential of the gate signal Clamp_D of
the first transistor NMOS0 changes to a potential higher than the
potential vblc+thn. This makes it possible to prevent excessive
charge of the bit line BL.
Arrangement Example
[0103] An arrangement example is practically the same as in the
first embodiment, and a detailed description thereof will not be
given.
<Data Read Operation>
[0104] A data read operation by a sense amplifier SA according to
the second embodiment will be described next with reference to
FIGS. 12 and 13. This description will be given with reference to a
timing chart shown in FIG. 12.
[0105] As shown in FIG. 12, first, at time t0 at which charge of
the bit line BL starts, that is, in the beginning of charge of the
bit line BL at the start of a read operation, a charge control
circuit 2-1 raises the potential of the gate signal Clamp_D of the
clamp transistor NMOS0 to (vsax+vthn) or (vsac+vthn), raises the
potential of the gate signal Clamp_E of the clamp transistor NMOS0a
(vblc+vthn), raises the potential of the gate signal Clamp_F of the
clamp transistor NMOS0b to (vsax+vthn), and raises the potential of
the gate signal SW of the switch transistor NMOS0c to
(vblc+vthn).
[0106] In this embodiment, at time t0 at which charge of the bit
line BL starts, the charging capacity of the clamp transistor NMOS0
is driven to a maximum degree to rapidly charge the bit line BL.
This makes it possible to rapidly charge the bit line BL0, as
indicated by a solid line in FIG. 12.
[0107] At time t1, the charge control circuit 2-1 drops only the
potential of the gate signal Clamp_D of the first transistor NMOS0
for clamping to (vblc+vthn). Again, at time t1, the charging
capacity of the first transistor NMOS0 for clamping is returned to
the second characteristic (N0) to charge the bit line BL0. The
first transistor NMOS0 is controlled so that the operating point is
determined by the second characteristic (N0).
[0108] The voltage relationship of the charge control circuit 2-1
at time t1 at which charge of the bit line DL ends is as shown in
FIG. 13. At time t1, switching elements S21-2, S22-1, S23-1, S24-1,
and S24-2 are turned off, and switching elements S21-1, S22-2, and
S23-2 are turned on in accordance with control signals SW1-1 to
SW3-2 from a control circuit 7, as shown in FIG. 13. The second
embodiment is different in this respect from the first embodiment.
Therefore, the gate signals Clamp_D, Clamp_E, Clamp_F, and SW can
have a voltage relationship as mentioned above.
[0109] At time t2 a predetermined period after time t1, the charge
control circuit 2-1 raises the potential of the gate signal Clamp_E
of the second transistor NMOS0a for clamping to (vsac+vthn),
maintains the potential of the gate signal Clamp_F of the third
transistor NMOS0b, and raises the potential of the gate signal SW
of the fourth transistor NMOS0c for switching to (vdd+vthn).
[0110] In this manner, the second embodiment is different from the
first embodiment in that in the former the transition period T1 is
set between times t1 and t2, as shown in FIG. 12.
[0111] At time t3, when a memory cell current Icell0 supplied by a
memory cell MC0 is to be sensed after the bit line BL0 is charged,
the charge control circuit 2-1 drops the gate signal SW to turn off
the fourth transistor NMOS0c for switching.
[0112] At time t4 a predetermined period (sense period T0) after
time t3, the charge control circuit 2-1 drops the gate signal
Clamp_F of the third transistor NMOS0b for clamping to turn it off
so as to determine the potential of a node node_sen, and a sense
operation is then performed.
<Effect>
[0113] As described above, according to the semiconductor memory
device and the control operation therefor according to the second
embodiment, at least effect (1) mentioned above can be obtained.
Further, according to this embodiment, effects (2) and (3) can be
obtained as well:
[0114] (2) It is possible to prevent excessive charge of the bit
line BL.
[0115] The nonvolatile memory device according to the second
embodiment controls the first to fourth transistors NMOS0 and
NMOS0a to NMOS0c so that the transition period T1 is set between
times t1 and t2. With this control, the potentials of the gate
signals Clamp_E, Clamp_F, and SW of the second to fourth
transistors NMOS0a, NMOS0b, and NMOS0c, respectively, other than
the first transistor NMOS0 can be prevented from becoming higher
than the potential vblc+vthn of the gate signal Clamp_D of the
first transistor NMOS0 before the potential of the gate signal
Clamp_D of the first transistor NMOS0 changes to a potential higher
than the potential vblc+vthn. This makes it possible to prevent
excessive charge of the bit line BL.
[0116] (3): This embodiment is advantageous to reduce the
manufacturing cost.
[0117] Further, in this embodiment, there is no need to add a new
constituent element to the arrangement in the first embodiment, so
the charge control circuit 2-1 need only be controlled so that the
transition period T1 is set. This makes it possible to reduce the
manufacturing cost.
Third Embodiment
Example in which EQL Period is Further Set in Transition Period
[0118] A semiconductor memory device and a control operation
therefor according to the third embodiment will be described next
with reference to FIGS. 14, 15, 16, 17, 18 and 19. The third
embodiment is different from the second embodiment in that in the
former a period (to be referred to as an EQL period hereinafter) to
short the nodes of gate signals Clamp_D, Clamp_E, and SW of first,
second, and fourth transistors is set in a transition period T1. A
detailed description of the same parts as in the second embodiment
will not be given hereinafter.
<NMOS Load Characteristic in Transition Period>
[0119] The load characteristic of an NMOS transistor during the
transition period T1 will be described first with reference to FIG.
14.
[0120] In the second embodiment, excessive charge of the bit line
BL can be prevented by setting the transition period T1.
[0121] However, the voltage relationship among the transistors
during the transition period T1 may exhibit the load characteristic
(N2) of the NMOS transistor, which expresses a driving capacity
lower than that expressed by the load characteristic (N1) of the
NMOS transistor.
[0122] That is, the first, second, and fourth transistors NMOS0,
NMOS0a, and NMOS0c applied with the lower potential (vblc+vthn) are
connected in series with each other, when viewed from the sense
amplifier SA. Therefore, the load characteristic of the sense
amplifier SA becomes the load characteristic (N2) poorer than the
original load characteristic (N1: second characteristic), and this
may lower the operating point of the bit line BL, as indicated by a
broken line in FIG. 14.
[0123] Hence, in the third embodiment, a period (EQL period) to
short the nodes of the gate signals Clamp_D, Clamp_E, and SW of the
first, second, and fourth transistors is further set in the
transition period T1 to generate an intermediate potential. This
makes it possible to bring the load characteristic (N1: second
characteristic) of a sense amplifier SA during the transition
period T1 close to the original load characteristic (N1: second
characteristic), and to prevent excessive charge of a bit line BL
upon a potential transition from the intermediate potential.
Arrangement Example
Bit Line Control Circuit
[0124] An example of the arrangement of a bit line control circuit
2 according to the third embodiment will be described next with
reference to FIG. 15.
[0125] The third embodiment is different from the second embodiment
in that in the former a charge control circuit 2-1 controls so that
a period (EQL period) to short the nodes of the gate signals
Clamp_D, Clamp_E, and SW of the first, second, and fourth
transistors is set in the transition period T1 to generate an
intermediate potential, as shown in FIG. 15.
[0126] Charge Control Circuit
[0127] An example of the arrangement of the charge control circuit
2-1 according to the third embodiment will be described next with
reference to FIG. 16.
[0128] The third embodiment is different form the second embodiment
in that in the former the charge control circuit 2-1 further
includes transistors EQL1, EQL2, and EQL3, as shown in FIG. 16.
[0129] The transistor EQL1 has a current path with its one end and
other end connected to the adjacent output terminals of the gate
signals Clamp_D and Clamp_E of the first and second transistors
NMOS0 and NMOS0a, respectively, and has its gate supplied with a
control signal SW4-1 output from a control circuit 7.
[0130] The current path of the transistor EQL2 has its one end and
other end connected to the adjacent output terminals of the gate
signals Clamp_E and Clamp_F of the second and third transistors
NMOS0a and NMOS0b, respectively, and has its gate supplied with a
control signal SW4-2 output from the control circuit 7.
[0131] The current path of the transistor EQL3 has its one end and
other end connected to the adjacent output terminals of the gate
signals Clamp_E and SW of the third and fourth transistors NMOS0b
and NMOS0c, respectively, and has its gate supplied with a control
signal SW4-3 output from the control circuit 7.
<Data Read Operation>
[0132] A data read operation by the sense amplifier SA according to
the third embodiment will be described next with reference to FIGS.
17 and 18. This description will be given with reference to a
timing chart shown in FIG. 17.
[0133] As shown in FIG. 17, first, at time t0, that is, in the
beginning of charge of the bit line BL at the start of a read
operation, the charge control circuit 2-1 raises the potential of
the gate signal Clamp_D of the first transistor NMOS0 for clamping
to (vsax+vthn) or (vsac+vthn), raises the potential of the gate
signal Clamp_E of the second transistor NMOS0a for clamping to
(vblc+vthn), raises the potential of the gate signal Clamp_F of the
third transistor NMOS0b for clamping to (vsax+vthn), and raises the
potential of the gate signal SW of the fourth transistor NMOS0c for
switching to (vblc+vthn).
[0134] Note that as described above, in the beginning of charge of
the bit line BL (time t0), the charging capacity of the clamp
transistor NMOS0 is driven to a maximum degree (gate signal
Clamp_D: vsax+vthn, vsac+vthn) to rapidly charge the bit line BL in
accordance with the NMOS characteristic (N1: first characteristic).
This makes it possible to increase the charging capacity so as to
rapidly charge a bit line BL0, as indicated by a broken line in
FIG. 17.
[0135] At time t1, the charge control circuit 2-1 drops only the
potential of the gate signal Clamp_D of the first transistor NMOS0
for clamping to (vblc+vthn). Again, at time t1, the bit line BL0 is
charged in accordance with the NMOS characteristic (N0: second
characteristic) obtained by returning the charging capacity of the
first transistor NMOS0 for clamping to the lower charging capacity,
thereby controlling so that the final operating point is determined
by the NMOS characteristic (N0: second characteristic).
[0136] In addition, in this embodiment, at this time, the charge
control circuit 2-1 sets a period (EQL period) T3 to short the
nodes of the gate signals Clamp_D, Clamp_E, and SW of the first,
second, and fourth transistors during the transition period to
generate an intermediate potential. This makes it possible to bring
the load characteristic of the sense amplifier SA during the
transition period T1 close to the original load characteristic (N1:
second characteristic), and to prevent excessive charge of the bit
line BL upon a potential transition from the intermediate
potential.
[0137] The voltage relationship of the charge control circuit 2-1
during the EQL period T3 is as shown in FIG. 18. During the EQL
period T3, all switching elements S21-1 to S24-2 are turned off in
accordance with control signals SW1-1 to SW3-2 and the control
signals SW4-1 to SW4-3 from the control circuit 7, and the
transistors EQL1 to EQL3 are turned on, as shown in FIG. 18. This
makes it possible to short the nodes of the gate signals Clamp_D,
Clamp_E, Clamp_F, and SW so as to obtain a voltage relationship
which generates an intermediate potential.
[0138] At time t2 after the elapse of the EQL period T3, the charge
control circuit 2-1 raises the potential of the gate signal Clamp_E
of the second transistor NMOS0a for clamping to (vsac+vthn),
maintains the potential of the gate signal Clamp_F of the third
transistor NMOS0b, and raises the potential of the gate signal SW
of the fourth transistor NMOS0c for switching to (vdd+vthn).
[0139] In this manner, the third embodiment is different from the
second embodiment in that in the former the transition period
defined between times t1 and t2 becomes the EQL period T3, as shown
in FIG. 17.
[0140] At time t3, when a memory cell current Icell0 supplied by a
memory cell MC0 is to be sensed after the bit line BL0 is charged,
first, the charge control circuit 2-1 drops the gate signal SW to
turn off the fourth transistor NMOS0c for switching.
[0141] At time t4 a predetermined period (sense period T0) after
time t3, the charge control circuit 2-1 drops the gate signal
Clamp_F of the third transistor NMOS0b for clamping to turn it off
so as to determine the potential of a node node_sen, and a sense
operation is then performed.
<Effect>
[0142] As described above, according to the semiconductor memory
device and the control operation therefor according to the third
embodiment, at least effects (1) to (3) mentioned above can be
obtained. Further, according to this embodiment, effect (4) can be
obtained as well:
[0143] (4) It is possible to suppress a decrease in load
characteristic of the sense amplifier SA during the transition
period so as to bring it close to the original load characteristic
(N1: second characteristic), and to prevent excessive charge of the
bit line BL.
[0144] As described above, the third embodiment is different from
the second embodiment in that in the former the charge control
circuit 2-1 controls so that the period (EQL period) to short the
nodes of the gate signals Clamp_D, Clamp_E, and SW of the first,
second, and fourth transistors is set in the transition period T1
to generate an intermediate potential. This makes it possible to
bring the load characteristic of the sense amplifier SA during the
transition period T1 close to the original load characteristic (N1:
second characteristic), and to prevent excessive charge of the bit
line BL upon a potential transition from the intermediate
potential.
[0145] As a result, the load characteristic according to this
embodiment is as shown in, for example, FIG. 19.
[0146] As is obvious from a solid line (N3: this embodiment) shown
in FIG. 19, in the third embodiment, it is possible to maintain a
given operating point using the load characteristic (N3: third
characteristic) obtained by bringing the load characteristic of the
sense amplifier SA during the transition period close to the
original load characteristic (N1: second characteristic), and to
prevent excessive charge of the bit line BL.
[0147] Note that the gate signal Clamp_D of the first transistor
NMOS0 has changed to the potential (vsax+vthn) or (vsac+vthn)
before the transition period of time, the potential of each node
always satisfies vblc+vthn<node potential<vsax(vsac)+vthn
during the EQL period T3. Accordingly, in theory, the sense
amplifier SA has a load characteristic equivalent to a series
circuit of the first to fourth transistors NMOS0, NMOS0a, NMOS0b,
and NMOS0c having an intermediate node (that is, has the load
characteristic between the first characteristic (N0) and the second
characteristic N1). However, in practice, it is obvious that the
sense amplifier SA has a load characteristic better than the series
circuit of the first, second, and fourth transistors NMOS0, NMOS0a,
and NMOS0c applied with the potential (vblc+vthn), as in the second
embodiment.
[0148] In addition, the third embodiment is more advantageous than
the first modification (to be described later) in that in the
former the current upon shorting can be decreased despite an
increase in number of switching elements which constitute the
charge control circuit 2-1. In this manner, the arrangement
according to the third embodiment is applicable as needed.
[First Modification (Another Example in Which EQL Period is Further
Set in Transition Period)]
[0149] A semiconductor memory device and a control operation
therefor according to the first modification will be described next
with reference to FIG. 20. The first modification relates to
another example in which a period (EQL period) to short the nodes
of gate signals Clamp_D, Clamp_E, and SW of the first, second, and
fourth transistors is set in the transition period. A detailed
description of the same parts as in the third embodiment will not
be given hereinafter.
Arrangement Example
Charge Control Circuit
[0150] FIG. 20 illustrates an example of the arrangement of a
charge control circuit 2-1 according to the first modification.
[0151] The first modification is different from the third
embodiment in that in the former the charge control circuit 2-1
includes no transistors EQL1 and EQL2, as shown in FIG. 20.
<Data Read Operation>
[0152] In a data read operation, a period (EQL period) T3 to short
the nodes of the gate signals Clamp_D, Clamp_E, and SW of the
first, second, and fourth transistors is similarly set in the
transition period (EQL period T3) to generate an intermediate
potential. This makes it possible to bring the load characteristic
of a sense amplifier SA during the transition period T3 close to
the original load characteristic (N1: second characteristic), and
to prevent excessive charge of a bit line BL upon a potential
transition from the intermediate potential.
[0153] The voltage relationship of the charge control circuit 2-1
during the EQL period T3 is as shown in FIG. 20. During the EQL
period T3, all switching elements S21-1 to S24-2 are turned off in
accordance with control signals SW1-1 to SW3-2 from a control
circuit 7, as shown in FIG. 20. This makes it possible to short the
nodes of the gate signals Clamp_D, Clamp_E, Clamp_F, and SW so as
to obtain a voltage relationship which generates an intermediate
potential.
<Effect>
[0154] As described above, according to the semiconductor memory
device and the control operation therefor according to the first
modification, at least effects (1) to (4) mentioned above can be
obtained.
[0155] Further, the first modification is more advantageous to
micropatterning than the third embodiment in that in the former the
charge control circuit 2-1 does not require transistors EQL1 and
EQL2, thereby reducing the number of switching elements which
constitute the charge control circuit 2-1. In this manner, the
arrangement according to the first modification is applicable as
needed.
[0156] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
embodiments described herein may be embodied in a variety of other
forms; furthermore, various omissions, substitutions and changes in
the form of the embodiments described herein may be made without
departing from the spirit of the inventions. The accompanying
claims and their equivalents are intended to cover such forms or
modifications as would fall within the scope and spirit of the
inventions.
* * * * *